It is common to form a solder mask of a resin composition for masks on a printed wiring board. Furthermore, when a printed wiring board having a through-hole or a via hole is used, the through-hole or the like is often filled with the resin composition for masks, in order to suppress deterioration in appearance of the resultant product due to a solder adhering to a periphery of the through-hole or the like, or due to a flux extending to a back side of the printed wiring board through the through-hole or the like.
When the resin composition for masks is used as described above, particularly when the resin composition for masks is used for filling the through-hole or the like, required are properties such as a property (crack resistance) that a crack is less likely to occur in a cured product of the resin composition in a process of post cure or solder leveling, a property (bulge resistance) that a bulge on the cured product and peeling due to the bulge are less likely to occur in a process of solder-leveling, and a property (protrusion resistance) that the cured product is less likely to protrude from the through-hole or the like in a process of post cure or solder leveling.
Conventionally, Patent Literature 1, for example, proposes, as an example of the resin composition for masks used for filling holes, a photosensitive resin composition capable of being developed by alkaline developers, the photosensitive resin composition containing: photosensitive resin having a carboxyl group obtained by reacting polybasic acid anhydride with a product of a reaction of an epoxy compound having a particular structure including epoxy groups at both ends with monocarboxylic acid having an unsaturated group; copolymer resin having a carboxyl group obtained by reacting (meth)acryl-based copolymer resin having a carboxyl group with a compound having both of an oxysilanyl ring and an ethylenically unsaturated group per molecule; a photopolymerization initiator; a reactive diluent; and a compound having an epoxy group. A through-hole or the like is filled with the photosensitive resin composition described in Patent Literature 1, and the photosensitive resin composition by means of irradiating it with activated energy rays and thereafter the photosensitive resin composition is subjected to two-stage or three-stage heating. Consequently, the through-hole or the like is filled.
However, techniques described in Patent Literature 1 require two-stage or three-stage heating for filling holes as described above, and have a problem of poor processing efficiency. In contrast, when only one-stage heating is performed in order to improve processing efficiency, thus-obtained cured product for filling the through-hole or the like has insufficient properties of the crack resistance, the bulge resistance, the protrusion resistance, and the like.